The present invention relates to improvements in rotary cutters for making holes in seams of ore, rock or the like, especially for gouging out ore in mine faces of underground excavations.
It is known to provide the rotary cutter of a mining machine with a front set of material removing bits which penetrate into a seam or body of rock or ore (hereinafter called ore for short) when the cutter is driven to rotate about is axis and is simultaneously moved forwardly toward and into the seam, e.g., into the face of a seam of coal at the front end of an underground excavation. Examples of mining machines which embody such cutters are so-called coal moles. Each cutter further carries a second set of bits which surround and are located rearwardly of the front bits to remove material from the seam when the cutter is moved sideways, i.e., radially of the hole which is formed by the front bits. As a rule, heretofore known cutters of the just outlined character resemble slender cones with rounded apices or truncated cones with slightly concave or convex front end faces (i.e., those end faces which are remote from the bases of the respective cones). The exterior of the cone carries the aformentioned front bits which remove material while the cutter penetrates into the seam. The bits of the second set are attached to the conical surface rearwardly of the apex of the cutter to remove material while the cutter moves sideways, e.g., up and down and horizontally back and forth, to thereby enlarge the hole which is formed by the front bits during initial penetration of the cutter into the seam. The (front) bits which are attached to and extend forwardly from the front end face also engage the material of the seam during lateral movement of the cutter. The axes of the front bits are nearly parallel to the axis of the cone, i.e., the angle between the axis of each such bit and the axis of the cone is a relatively small acute angle. The peripheral speed of such bits is relatively low and the torque which is applied thereto is substantial. This brings about a number of drawbacks. First of all, the stresses upon the front bits which are nearly parallel with and close to the axis of the conical cutter are very pronounced; therefore, these bits are subjected to extensive wear and are heated to elevated temperatures during removal of material. This causes their material (normally hard steel) to become brittle and to break after relatively short periods of use. The same holds true for the holders which connect the just discussed bits to the body of the conical cutter. It has been found that the useful life of holders for the front bits which are close to the axis of the cutter, i.e., for the bits which are located at the forward end of the conical body of the cutter, is extremely short. Consequently, the cutter must be dismantled and its front bits as well as their holders replaced at frequent intervals with attendant losses in output. The dismantling and overhauling of cutters must be carried out in a workshop.
Secondly, the front bits whose peripheral speed is low tend to jam in the material of the seam. Once a front bit jams, the cutter tends to rotate about the jammed front bit rather than about the axis of its conical body. This will be readily appreciated by hearing in mind that the axes of bits which are located at the forward end of the cutter are nearly parallel to the axis of the conical body. Since the cutter is normally mounted at the forward end of an elongated arm or boom, jamming of a front bit and the resulting rotation of the cutter about the jammed bit entails pronounced vibration and other undesirable stray movements of the entire boom as well as of other parts of the mining machine with well-known undesirable consequences including noise, excessive wear and tendency to break.